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A friend has decided, due to warming temperatures, to reduct the gas flow to 80% of normal by partially closing the main shutoff valve. He has a natural draft furnace and says the flame is about 3/4 the height when the main gas valve is fully opened. The furnace now works perfectly with the gas flow reduction, other than a longer cycle time, which was his goal.

A couple of things come to mind, this being a natural draft furnace. If the gas flow was reduced, was the air mixture also adjusted? Could he be setting himself up for sooting problems (would require more fuel than air)? Condensation in the heat exchanger due to cooler operating temperatures?

Wendel,your friend has just increased the cost of his gas bill & probably CO production in his furnace by reducing the gas pressure by 80%.
He should have it tested by someone who has been certified in carbon monoxide testing to insure that he has not created an unsafe condition.

how does you friend know it's 80% maybe it's 60 or 70%. Call your frend and tell him in theory it's a good idea. but he's an a-hole and dangering the lives of anyone who enter his house. Next time you see him take the palm of your hand and jam it into his forehead and knock some sense into him.

I agree with all above. The first question that comes to mind is why are you throttling the gas shut off valve in the first place. Was it, as mentioned, short cycling? In which case that problem needs to be addressed. There is now way you can quess that the Btu input is at 80% unless someone has check input by clocking the gas meter. You need to contact a professional Hvac person as soon as possible and this forum is not the place. Your asking for a lot more trouble operating the unit in this condition than before. They put limit switches and safety switches in the circuit for a good reason. What do you guys say?

from the original post it doesn't sound like there was a problem that he was trying to fix, just an attempt to reduce the amount of gas that the furnace was using when it wasn't so cold out anymore. Thing is that if it's a furnace like the one i've got then it's already got a thermostat connected to it, so if it's warm enough out it's not going to run anyway.

Had a similar discussion with the baby sitter here last week, she came to me to tell me that she had turned off the electric heater that we have in the baby's room because it was nice and warm in there. 1, the reason it was nice and warm was because of the heater, and the heater has a thermostat so when it gets warm it will SHUT OFF. just like the furnace which will not run when it's warm in the house.

walk your friend through this logic. If you reduce the heat output of the furnace by half, then the thermostat will have to run it twice as long to get the same heat out of it. So he's using the same amount of gas, just over a longer period of time. in reality it's worse than that and on top all the safety concerns that others have already brought up.

Thank you all for your insightful comments. I passed them along and although he didn't have a CO measurement because the furnace is in an unheated, well-ventilated attic, he did say he used a pressure gauge to set the burner side of the furnace gas valve pressure to 3.0" W.C. The furnace data plate calls for 3.5" W.C. The pressure change (3.5 to 3.0) is how he arrived at the 20% less heat from the burners. He seems to have been careful in trying to lessen the short cycles when it's not very cold out.

Suprises me there is no delayed ignition. Especially if there is a pilot. If he figured out how to take manifold pressure why couldn't he have adjusted on reg? Although we know HE shouldn't adjust at all.

Let's not forget about stack temperature too. Also lowering delte t also lowers effiency and who knows what kind of co levels there are. Not a good idea at all. Open the cock up and have the furnace tuned by a pro, that will save him money and keep him safe.
Brian

It may not make a difference at all...it depends on how far your furnace is from the meter...it seems to me if your buddy is cutting the valve off a percentage, he is restricting the flow through the pipe...usually the gas line is pretty oversized to begin with...he probably has accomplished nothing, other than setting himself up for a service call for a furnace that's all sooted up, or a potentially explosive situation. I would recommend that he open the valve fully, and then call a qualified person to give him a quote on a two speed furnace...the math was already figured out for him by someone who already has seen this situation before. Good-Luck.

I just re-read a previous post that stated the manifold pressure was de-rated to 3.0" hg. This is an explosion waiting to happen...have you ever seen delayed ignition on a gas furnace....can you say BOOM!!! Just wanted to edit this response so there isn't any misunderstanding, get your buddy some help before he kills someone!!!! I'm very serious here. Good-Luck!